Network Layer4-1 Chapter 4 Network Layer Part 2: IP: Internet Protocol Computer Networking: A Top...

Network Layer 4-1

Chapter 4Network Layer

Part 2:IP: Internet Protocol

Computer Networking: A Top Down Approach 6th edition Jim Kurose, Keith RossAddison-WesleyMarch 2012

Network Layer 4-2

Chapter 4: Network Layer

4. 1 Introduction 4.2 Virtual circuit and datagram networks 4.3 What’s inside a router 4.4 IP: Internet Protocol

Datagram format IPv4 addressing ICMP IPv6

4.5 Routing algorithms Link state Distance Vector Hierarchical routing

4.6 Routing in the Internet RIP OSPF BGP

4.7 Broadcast and multicast routing

Network Layer 4-3

The Internet Network layer

forwardingtable

Host, router network layer functions:

Routing protocols• path selection• RIP, OSPF, BGP

IP protocol• addressing conventions• datagram format• packet handling conventions

ICMP protocol• error reporting• router

“signaling”

Transport layer: TCP, UDP

Link layer

physical layer

Networklayer

Network Layer 4-4







Network Layer 4-5

IP datagram format

ver length

32 bits (same for IPv6 )

data (variable length,typically a TCP

or UDP segment)

16-bit identifier

header checksum

time tolive

32 bit source IP address (128bits in IPv6)

IP protocol versionnumber (4 bits)

header length ( in bytes)(4 bits)

max numberremaining hops

(decremented by 1 at each router; die at 0)

forfragmentation/Reassembly; see next slide

total datagramlength (bytes) (this field is 16 bits)

upper layer protocolto deliver payload to;

value of 6 = TCP, 17 = UDP

head.len

type ofservice

“type” of data flgsfragment

offsetupper layer

32 bit destination IP address (128bits in IPv6)

Options (if any) E.g. timestamp,record routetaken, specifylist of routers to visit.

how much overhead with TCP?

20 bytes of TCP 20 bytes of IP

(no options) = 40 bytes + app

layer overhead

16 bits, so max data is 6,535 bytes. Typical size is 1,500bytes

Network Layer 4-6

IPv6 Header (Cont)Priority: identify priority among datagrams in flowFlow Label: identify datagrams in same “flow.” (concept of“flow” not well defined).Next header: identify upper layer protocol for data

Network Layer

Header checksum

Used by routers Computed by treating each 2 bytes in the

header as a number and sum these using 1s complement.

Store in the checksum field Router computes header checksum for each IP

datagram Discard datagram if checksum is wrong

Checksum must be recomputed and stored again at every router; TTL and maybe options fields change

4-7

Network Layer

Header checksum

Why are there checksums at both TCP and IP level? IP only check IP header, TCP/UDP checksum

entire TCP/UDP segment TCP/UDP do not have to run on IP (eg could

run on ATM)

4-8

Network Layer 4-9

IP Fragmentation & Reassembly network links have MTU

(max.transfer size) - largest possible link-level frame. different link types,

different MTUs* large IP datagram divided

(“fragmented”) within net one datagram becomes

several datagrams “reassembled” only at

final destination IP header bits used to

identify, order related fragments

fragmentation: in: one large datagramout: 3 smaller datagrams

reassembly

*Example: Ethernet up to 1,500 bytes, some wide-area links up to 576Problem: different links from host-to-host may by diff types!

Network Layer 4-10

IP Fragmentation & Reassembly Fragmentation not allowed

in IPv6 if fragment is too big for a

router it throws it away and sends an ICMP message back to the sender

now sender is responsible for datagram size, not routers

simplifies router’s job, more efficient overall

fragmentation: in: one large datagramout: 3 smaller datagrams

reassembly

Network Layer 4-11

IP Fragmentation and Reassembly

ID=x

offset=0

fragflag=0

length=4000

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=185

fragflag=1

length=1500

ID=x

offset=370

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams

Example 4000 byte

datagram MTU = 1500

bytes

1480 bytes in data field

offset = multiple of 8 bytes so1480/8 = 185

ID: set by sending host IP layer; typically increments ID num for each datagram it sends. Last fragment sent has flag field set to 0 to indicate it’s the last fragment; all other fragments have flag set to 1

If one fragment is lost,,IP discards all fragments

recall: header takes 20 bytes

Network Layer

Fragmentation costs

Complicates routers and end systems DoS attacks

Attacker sends series of bizarre fragments Jolt2 attack: attacker sends a stream of small

fragments to target host. None has offset of 0. Target collapses as it attempts to rebuild datagrams.

Another attack: send overlapping fragments. OS can crash attempting to reassemble.

IP v6 has no fragments. Fragmentation animation: http://www.awl.com/kurose-ross

4-12

Network Layer 4-13







Network Layer 4-14

IP Addressing: introduction IP address: 32-bit

identifier for host, router interface

interface: connection between host/router and physical link router’s typically

have multiple interfaces

host typically has one interface

IP addresses associated with each interface

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

223.1.1.1 = 11011111 00000001 00000001 00000001

223 1 11

Network Layer 4-15

IP addressing: introduction

Q: how are interfaces actually connected?A: we’ll learn about that in chapter 5, 6.

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A: wired Ethernet interfaces connected by Ethernet switches

A: wireless WiFi interfaces connected by WiFi base station

For now: don’t need to worry about how one interface is connected to another (with no intervening router)

Network Layer 4-16

Subnets IP address:

subnet part (high order bits)

host part (low order bits)

What’s a subnet ? device interfaces

with same subnet part of IP address

can physically reach each other without intervening router

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

network consisting of 3 subnets

subnet

Network Layer 4-17

Subnets 223.1.1.0/24223.1.2.0/24

223.1.3.0/24

Recipe To determine the

subnets, detach each interface from its host or router, creating islands of isolated networks.

Each isolated network is called a subnet. Subnet mask: /24

(leftmost 24 bits define the subnet address)

Network Layer 4-18

SubnetsHow many? 223.1.1.1

223.1.1.3

223.1.1.4

223.1.2.2223.1.2.1

223.1.2.6

223.1.3.2223.1.3.1

223.1.3.27

223.1.1.2

223.1.7.0

223.1.7.1223.1.8.0223.1.8.1

223.1.9.1

223.1.9.2

6

disconnect each interface on a router to find the subnets

one subnet (223.1.9)

each interface on the net has different address (.1 and .2)

Network Layer 4-19

IP addressing: CIDR

CIDR: Classless InterDomain Routing pronounced “cider” subnet portion of address of arbitrary length address format: a.b.c.d/x, where x is # bits in

subnet portion of address

11001000 00010111 00010000 00000000

subnetpart

hostpart

200.23.16.0/23

Network Layer 4-20

IP addresses: how to get one?

Q: How does a host get IP address?

hard-coded by system admin in a file Windows: control-panel->network-

>configuration->tcp/ip->properties UNIX: /etc/rc.config

DHCP: Dynamic Host Configuration Protocol: dynamically get address from as server “plug-and-play”

Network Layer 4-21

DHCP: Dynamic Host Configuration Protocol

Goal: allow host to dynamically obtain its IP address from network server when it joins networkCan renew its lease on address in useAllows reuse of addresses (only hold address while connected/ “on”)Support for mobile users who want to join network (more shortly)

DHCP overview: host broadcasts “DHCP discover” msg [optional] using

broadcast address 255.255.255.255 (source address 0.0.0.0) DHCP server responds with “DHCP offer” msg [optional]

using broadcast address 255.255.255.255 host requests IP address: “DHCP request” msg DHCP server sends address: “DHCP ack” msg

Network Layer 4-22

DHCP client-server scenario

223.1.1.0/24

223.1.2.0/24

223.1.3.0/24

223.1.1.1

223.1.1.3

223.1.1.4 223.1.2.9

223.1.3.2223.1.3.1

223.1.1.2

223.1.3.27223.1.2.2

223.1.2.1

DHCPserver

arriving DHCPclient needs address in thisnetwork

Network Layer 4-23

DHCP server: 223.1.2.5 arriving client

DHCP discover

src : 0.0.0.0, 68 dest.: 255.255.255.255,67DHCPDISCOVERyiaddr: 0.0.0.0transaction ID: 654

DHCP offer

src: 223.1.2.5, 67 dest: 255.255.255.255, 68DHCPOFFERyiaddrr: 223.1.2.4transaction ID: 654DHCP server Id: 223.1.2.5lifetime: 3600 secs

DHCP request

src: 0.0.0.0, 68 dest:: 255.255.255.255, 67DHCPREQUESTyiaddrr: 223.1.2.4transaction ID: 655DHCP server ID: 223.1.2.5lifetime: 3600 secs

DHCP ACK

src: 223.1.2.5, 67 dest: 255.255.255.255, 68DHCPPACKyiaddrr: 223.1.2.4transaction ID: 655DHCP server ID: 223.1.2.5lifetime: 3600 secs

DHCP client-server scenario

there may be several DHCP servers on this network, so client may get several offers.

transaction ID used to allow the client to identify the broadcast message

Network Layer 4-24

DHCP: more than IP address

DHCP can return more than just allocated IP address on subnet: address of first-hop router for client name and IP address of DNS sever network mask (indicating network versus

host portion of address)

Network Layer 4-25

DHCP: example

connecting laptop needs its IP address, addr of first-hop router, addr of DNS server: use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP, encapsulated in IP, encapsulated in 802.1 Ethernet Ethernet frame broadcast (dest: FFFFFFFFFFFF) on LAN, received at router running DHCP server

Ethernet demux’ed to IP demux’ed, UDP demux’ed to DHCP

168.1.1.1

Network Layer 4-26

DCP server formulates DHCP ACK containing client’s IP address, IP address of first-hop router for client, name & IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

encapsulation of DHCP server, frame forwarded to client, demux’ing up to DHCP at client

client now knows its IP address, name and IP address of DSN server, IP address of its first-hop router

DHCP: example

Network Layer 4-27

DHCP: wireshark output (home LAN)

Message type: Boot Reply (2)Hardware type: EthernetHardware address length: 6Hops: 0Transaction ID: 0x6b3a11b7Seconds elapsed: 0Bootp flags: 0x0000 (Unicast)Client IP address: 192.168.1.101 (192.168.1.101)Your (client) IP address: 0.0.0.0 (0.0.0.0)Next server IP address: 192.168.1.1 (192.168.1.1)Relay agent IP address: 0.0.0.0 (0.0.0.0)Client MAC address: Wistron_23:68:8a (00:16:d3:23:68:8a)Server host name not givenBoot file name not givenMagic cookie: (OK)Option: (t=53,l=1) DHCP Message Type = DHCP ACKOption: (t=54,l=4) Server Identifier = 192.168.1.1Option: (t=1,l=4) Subnet Mask = 255.255.255.0Option: (t=3,l=4) Router = 192.168.1.1Option: (6) Domain Name Server Length: 12; Value: 445747E2445749F244574092; IP Address: 68.87.71.226; IP Address: 68.87.73.242; IP Address: 68.87.64.146Option: (t=15,l=20) Domain Name = "hsd1.ma.comcast.net."

reply

Message type: Boot Request (1)Hardware type: EthernetHardware address length: 6Hops: 0Transaction ID: 0x6b3a11b7Seconds elapsed: 0Bootp flags: 0x0000 (Unicast)Client IP address: 0.0.0.0 (0.0.0.0)Your (client) IP address: 0.0.0.0 (0.0.0.0)Next server IP address: 0.0.0.0 (0.0.0.0)Relay agent IP address: 0.0.0.0 (0.0.0.0)Client MAC address: Wistron_23:68:8a (00:16:d3:23:68:8a)Server host name not givenBoot file name not givenMagic cookie: (OK)Option: (t=53,l=1) DHCP Message Type = DHCP RequestOption: (61) Client identifier Length: 7; Value: 010016D323688A; Hardware type: Ethernet Client MAC address: Wistron_23:68:8a (00:16:d3:23:68:8a)Option: (t=50,l=4) Requested IP Address = 192.168.1.101Option: (t=12,l=5) Host Name = "nomad"Option: (55) Parameter Request List Length: 11; Value: 010F03062C2E2F1F21F92B 1 = Subnet Mask; 15 = Domain Name 3 = Router; 6 = Domain Name Server 44 = NetBIOS over TCP/IP Name Server ……

request

Network Layer 4-28

IP addresses: how to get one?

Q: How does network get subnet part of IP addr?

A: gets allocated portion of its provider ISP’s address space

ISP's block 11001000 00010111 00010000 00000000 200.23.16.0/20

Organization 0 11001000 00010111 00010000 00000000 200.23.16.0/23 Organization 1 11001000 00010111 00010010 00000000 200.23.18.0/23 Organization 2 11001000 00010111 00010100 00000000 200.23.20.0/23 ... ….. …. ….

Organization 7 11001000 00010111 00011110 00000000 200.23.30.0/23

Network Layer 4-29

Hierarchical addressing: route aggregation

“Send me anythingwith addresses beginning 200.23.16.0/20”

200.23.16.0/23

200.23.18.0/23

200.23.30.0/23

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us“Send me anythingwith addresses beginning 199.31.0.0/16”

200.23.20.0/23Organization 2

...

...

Hierarchical addressing allows efficient advertisement of routing information:

Network Layer 4-30

Hierarchical addressing: more specific routes

ISPs-R-Us has a more specific route to Organization 1

“Send me anythingwith addresses beginning 200.23.16.0/20”

200.23.16.0/23

200.23.18.0/23

200.23.30.0/23

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us“Send me anythingwith addresses beginning 199.31.0.0/16or 200.23.18.0/23”

200.23.20.0/23Organization 2

...

...

Network Layer 4-31

IP addressing: the last word...

Q: How does an ISP get block of addresses?

A: ICANN: Internet Corporation for Assigned Names and Numbers

allocates addresses manages DNS assigns domain names, resolves disputes

Network Layer 4-32

NAT: Network Address Translation

10.0.0.1

10.0.0.2

10.0.0.3

10.0.0.4

138.76.29.7

local network(e.g., home network)

10.0.0/24

rest ofInternet

Datagrams with source or destination in this networkhave 10.0.0/24 address for

source, destination (as usual)

All datagrams leaving localnetwork have same single source

NAT IP address: 138.76.29.7,different source port numbers

Network Layer 4-33


Motivation: local network uses just one IP address as far as outside world is concerned: range of addresses not needed from ISP: just one

IP address for all devices can change addresses of devices in local network

without notifying outside world can change ISP without changing addresses of

devices in local network devices inside local net not explicitly

addressable, visible by outside world (a security plus).

Network Layer 4-34

NAT: Network Address TranslationImplementation: NAT router must:

outgoing datagrams: replace (source IP address, port #) of every outgoing datagram to (NAT IP address, new port #). . . remote clients/servers will respond using

(NAT IP address, new port #) as destination addr.

remember (in NAT translation table) every (source IP address, port #) to (NAT IP address, new port #) translation pair

incoming datagrams: replace (NAT IP address, new port #) in dest fields of every incoming datagram with corresponding (source IP address, port #) stored in NAT table

Network Layer 4-35


10.0.0.1

10.0.0.2

10.0.0.3

S: 10.0.0.1, 3345D: 128.119.40.186, 80

1

10.0.0.4

138.76.29.7

1: host 10.0.0.1 sends datagram to 128.119.40.186, 80

NAT translation tableWAN side addr LAN side addr

138.76.29.7, 5001 10.0.0.1, 3345…… ……

S: 128.119.40.186, 80 D: 10.0.0.1, 3345

4

S: 138.76.29.7, 5001D: 128.119.40.186, 80

2

2: NAT routerchanges datagramsource addr from10.0.0.1, 3345 to138.76.29.7, 5001,updates table

S: 128.119.40.186, 80 D: 138.76.29.7, 5001

3

3: Reply arrives dest. address: 138.76.29.7, 5001

4: NAT routerchanges datagramdest addr from138.76.29.7, 5001 to 10.0.0.1, 3345

Network Layer 4-36


16-bit port-number field: 60,000 simultaneous connections with a

single LAN-side address! NAT is controversial:

routers should only process up to layer 3 violates end-to-end argument

• NAT possibility must be taken into account by app designers, eg, P2P applications

address shortage should instead be solved by IPv6

Network Layer 4-37

NAT traversal problem client wants to connect to

server with address 10.0.0.1 server address 10.0.0.1

local to LAN (client can’t use it as destination addr)

only one externally visible NATted address: 138.76.29.7

solution 1: statically configure NAT to forward incoming connection requests at given port to server e.g., (123.76.29.7, port

2500) always forwarded to 10.0.0.1 port 25000

10.0.0.1

10.0.0.4

NAT router

138.76.29.7

Client?

Network Layer 4-38

NAT traversal problem solution 2: Universal Plug

and Play (UPnP) Internet Gateway Device (IGD) Protocol. Allows NATted host to: learn public IP address

(138.76.29.7) add/remove port

mappings (with lease times)

i.e., automate static NAT port map configuration

10.0.0.1

10.0.0.4

NAT router

138.76.29.7

IGD

Network Layer 4-39

NAT traversal problem solution 3: relaying (used in Skype)

NATed client establishes connection to relay External client connects to relay relay bridges packets between to

connections

138.76.29.7

Client

10.0.0.1

NAT router

1. connection torelay initiatedby NATted host

2. connection torelay initiatedby client

3. relaying established

Network Layer 4-40







Network Layer 4-41

ICMP: Internet Control Message Protocol

used by hosts & routers to communicate network-level information error reporting: unreachable host, network, port, protocol echo request/reply (used by ping)

network-layer “above” IP: ICMP msgs carried in IP datagrams

ICMP message: type, code plus first 8 bytes of IP datagram causing error

Type Code description0 0 echo reply (ping)3 0 dest. network unreachable3 1 dest host unreachable3 2 dest protocol unreachable3 3 dest port unreachable3 6 dest network unknown3 7 dest host unknown4 0 source quench (congestion control - not used)8 0 echo request (ping)9 0 route advertisement10 0 router discovery11 0 TTL expired12 0 bad IP header

Network Layer 4-42

Traceroute and ICMP

Source sends series of UDP segments to dest First has TTL =1 Second has TTL=2, etc. Unlikely port number

When nth datagram arrives to nth router: Router discards datagram And sends to source an ICMP message (type 11, code 0) Message includes name of router& IP address

When ICMP message arrives, source calculates RTT Traceroute does this 3 timesStopping criterion UDP segment eventually arrives at destination host Destination returns ICMP “host unreachable” packet (type 3,

code 3) When source gets this ICMP, stops.

3 probes 3 probes

Network Layer 4-43







Network Layer 4-44

IPv6 Initial motivation: 32-bit address space

soon to be completely allocated. Additional motivation:

header format helps speed processing/forwarding

header changes to facilitate QoS

IPv6 datagram format: fixed-length 40 byte header no fragmentation allowed

Network Layer 4-45

IPv6 Header (Cont)Priority: identify priority among datagrams in flowFlow Label: identify datagrams in same “flow.” (concept of“flow” not well defined).Next header: identify upper layer protocol for data

Network Layer 4-46

Other Changes from IPv4

Checksum: removed entirely to reduce processing time at each hop

Options: allowed, but outside of header, indicated by “Next Header” field

ICMPv6: new version of ICMP additional message types, e.g. “Packet Too

Big” multicast group management functions

Network Layer 4-47

Transition From IPv4 To IPv6

Not all routers can be upgraded simultaneous no “flag days” How will the network operate with mixed IPv4 and IPv6

routers? Tunneling: IPv6 carried as payload in IPv4

datagram among IPv4 routers

IPv4 source, dest addr IPv4 header fields

IPv4 datagram

IPv6 datagram

IPv4 payload

UDP/TCP payload

IPv6 source dest addrIPv6 header fields

Network Layer 4-48

TunnelingA B E F

IPv6 IPv6 IPv6 IPv6

tunnelLogical view:

Physical view:A B E F

IPv6 IPv6 IPv6 IPv6IPv4 IPv4

Network Layer 4-49

TunnelingA B E F

IPv6 IPv6 IPv6 IPv6

tunnelLogical view:

Physical view:A B E F

IPv6 IPv6 IPv6 IPv6

C D

IPv4 IPv4

Flow: XSrc: ADest: F

data


data


data

Src:BDest: E


data

Src:BDest: E

A-to-B:IPv6

E-to-F:IPv6

B-to-C:IPv6 inside

IPv4

B-to-C:IPv6 inside

IPv4

Network Layer4-1 Chapter 4 Network Layer Part 2: IP: Internet Protocol Computer Networking: A Top...

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